Yanahara Hot Spring is one of the strongest acid vitriol springs in the world. It contains 0.36 g of free hydrogen ion, 14.6 g of ferrous ion, 0.8 g of ferric ion, and 0.44 g of copper in one liter. Its internal use has an remarkable effect on the recovery of hypochromic anaemia. Ten ml of this vitriol water was diluted with plain water to two hundred ml and was given to twenty six patients by stomach tube at 37℃. Gastric acidity of the patients was measured at thirty minutes intervals during the following 150 minutes after the administration of the water. In seven cases of normoacidity no significant difference was proved between the effects of the mineral water and of caffeine control (0.2 g in 200 ml.). In ten cases of hyperacidity a marked decrease in the free gastric acidity curve was recognized by the use of the mineral water compared with caffeine control. In nine cases of hypo- and anacidity both free and total acidity curve was elevated by the use of the mineral water compared with the curve of caffeine control.